Patent GB-783773-A (GB)
Title: Improvements in and relating to mercury vapour electric discharge apparatus
IPC: H01J13/04
Filed: 1952-06-05
Abstract: 783,773. Discharge apparatus. BERTELE, H. C. May 28 1953 [June 5, 1952], No. 14217/52. Class 39(1). Electric discharge apparatus containing an atmosphere of mercury vapour produced by vaporization from a reservoir of liquid mercury has a cathode comprising a thin film of mercury directly on an impervious member of arc resisting metal, the temperature of the cathode in operation being effectively below the saturation temperature of the mercury vapour atmosphere and the temperature of the envelope being effectively above the saturation temperature of the mercury vapour atmosphere and the arrangement being such that the film of mercury is maintained by condensation of mercury vapour from the atmosphere directly on to the cathode throughout the whole of the current range of the apparatus. The cathode with the film of mercury can be formed in a discharge apparatus for example as shown in Fig. 7, in which a cathode cap 13 of molybdenum or tungsten 11 just covered by the annular mercury reservoir is. Air is evacuated from the envelope 10 by a vacuum pump 56 and then a flow of cooling liquid 57, 58 is established and the starter electrode 21 operated to establish a discharge which is transferred to the anode 16 by applying a suitable potential to the control electrode 18. Pumping is continued to remove gases emanating from the various metals within the envelope and later to extract mercury vapour from the envelope which is condensed in the condenser 59. As the mercury vapour is extracted the level of the liquid mercury 11 falls until it reaches the surface of the cathode cap 13, a small spot on the cap becoming wetted by the mercury under the influence of the discharge. Spreading of the spot to cover if necessary the whole of the cap 13 is effected by gradually increasing the current. In a modification the discharge is established between a small spot of mercury on the cap 13, a recess therefor being provided if necessary, the extraction of mercury being not then required. In another modification shown in Fig. 10, the cathode 13 is cooled and by the starter electrode 60 a discharge is established between the anode and the surface of a pool 11 of mercury in contact with the cathode 13, the mercury wetting the rim of the cathode 13 and the wetted area spreading over the surface thereof. In the discharge apparatus shown in Fig. 1, a glass envelope 10 has a cup-shaped base containing a pool of mercury 11 and a rod 12 of molybdenum or tungsten, having a film of mercury 13 thereon which can be formed during manufacture, projecting above the surface of the pool, the end 14 of the rod being provided with cooling fins 15 over which air may be blown. A control electrode 18 is disposed between the anode 16 and the rod 12 and a starter electrode 21 is mounted on a spring 22, an electromagnet being provided for raising and lowering the starter electrode. A shroud 25 is provided for ensuring the required temperature conditions. The control electrode 18 may also be employed as a keep-alive electrode. In another construction, shown in Fig. 2, the majority of the envelope is formed of a steel cylinder 16 constituting the anode and the cathode 12 is of copper metallically bonded into a steel sleeve 26 sealed into a glass insulator 27 sealed into a sleeve 28. A cap 13 of arc resisting metal is formed on top of the combined rod 12 and sleeve 26. A ceramic ring 29 forms a reservoir for the mercury 11. The starter electrode 21 is carried by a rod 34 having a soft iron armature 35 connected by a spring 22 fixed to a stainless steel cylinder 32 surrounded by an electromagnet 33. The anode cylinder 16 is surrounded by a copper sleeve 36 having cooling fins 37. A central portion of the sleeve 36 is spaced by an air gap of a few thousandths of an inch from the cylinder 16. A thin film of mercury is formed on the cap 13 during manufacture. A stream of air is directed upwardly over the fins 15 and 37. The control electrode may be used as a keep-alive electrode. In the construction shown in Fig. 3 having a glass envelope 10 the molybdenum cap 13 is cooled by a copper block 12 having fins 15 over which a stream of air is blown, the upper part of the apparatus is heated by the heater 41 and the discharge space is restricted by the shield 40 close to the envelope. Baffles 61 are provided to prevent direct radiation of heat from the control electrode 18 and anode 16 on to the cap 13. The starter electrode is a hinged flap 21 whose free end is normally in contact with the film of mercury on the cap 13 and which is drawn away from the film during starting by the electromagnet 33. A keep-alive electrode 62 is provided. The cathode may be water-cooled. In the construction shown in Fig. 5, the anode 16 is of graphite, the control electrode 18 is in the form of six frusto-conical members forming a baffle and the rod for the electromagnetically operated starter electrode 21 is connected to metal bellows 46. The cathode is water cooled and the envelope 10 is cooled by passing the water from the outlet port 44 through a valve 49 to an inlet port 50 of the jacket 47 and to an outlet port 50<SP>1</SP>. The valve 49 is normally closed and is opened by the thermostat 48 when the temperature of the water in the jacket 47 exceeds a predetermined value. In the construction shown in Fig. 6 the cathode comprises a steel cylinder 53 covered by a molybdenum layer 13 surrounded by a control electrode 18 and a steel cylindrical anode 16. The ends of the cylinders 53 and 16 are closed by glass seals 54 and 55, the seal 55 having an annular reservoir for the mercury 11. A movable starter electrode 21 is provided and a stream of cooling air or water is passed through the bore of the cathode. Other starting arrangements can be used. In the capacitive starting arrangement shown in Fig. 11, the starter electrode 21 has a domed lower end having a radius of curvature slightly less than that of a semispherical recess in the cap 13 to provide a small gap between the electrode 21 and the thin film of mercury on the cap 13. A keep-alive electrode 62 closely surrounds the starter electrode to protect it from the main discharge. The main discharge can be caused to move away from the starter electrode as described in Specification 670,417. In the Ignitron method of starting shown in Fig. 12 a semi-conductor element 21 is tapered to fit into a recess in the cap 13 and contact the film of mercury thereon. In all constructions the top surface of the cathode can be flat or convex. Specification 670,416 is referred to. The Provisional Specification states that mercury can be replaced by a metal solid at room temperatures but liquid at operating temperatures. Specification 22246/12 [Class 39], is referred to.